1. Field of the Invention
The present invention relates to a system for producing a polycondensation polymer. More particularly, the present invention is concerned with a system for producing a polycondensation polymer, comprising (A) an inert gas absorption device for causing a molten polycondensation prepolymer to absorb an inert gas to thereby obtain (.alpha.) a molten prepolymer having the inert gas absorbed therein, (B) a polymerizer device for polymerizing the inert gas-absorbed molten prepolymer (.alpha.) under a reduced pressure, and (C) a pipe for transferring the inert gas-absorbed molten prepolymer (.alpha.) from absorption device (A) to polymerizer device (B), wherein absorption device (A) and polymerizer device (B) are arranged in this order and connected to each other through pipe (C). The present invention is also concerned with a method for producing a polycondensation polymer, which comprises: treating a molten polycondensation prepolymer with an inert gas in an inert gas absorption zone to cause the molten polycondensation prepolymer to absorb the inert gas, thereby obtaining an inert gas-absorbed molten prepolymer (.alpha.); subsequently transferring the inert gas-absorbed molten prepolymer (.alpha.) to a polymerization reaction zone; and subjecting the inert gas-absorbed molten prepolymer (.alpha.) to polymerization in the polymerization reaction zone under a specific reduced pressure, to thereby polymerize the inert gas-absorbed molten prepolymer (.alpha.) to a predetermined degree of polymerization. By the use of the system or the method of the present invention, it has become possible to produce a colorless, high quality polycondensation polymer at high polymerization rate even without using a large amount of an inert gas.
2. Prior Art
As methods for producing polycondensation polymers, such as polyesters, polyamides and polycarbonates, there are known an interfacial polycondensation process which uses a solvent and a melt polycondensation process which uses no solvent. In general, a process which uses a solvent has problems in that not only is a great deal of labor required to remove the solvent from the produced polymer, but also a small amount of the solvent remaining in the produced polymer adversely affects the properties of the polymer.
In the case of a melt polycondensation process, a polycondensation reaction involved in this process is an equilibrium reaction and, hence, a polycondensation polymer is generally produced while removing by-product(s) formed in the polycondensation reaction from the equilibrium polycondensation reaction system so as to displace the equilibrium to the product side. For example, water and ethylene glycol are removed from the reaction system in the production of polyethylene terephthalate, and water is removed from the reaction system in the production of hexamethylene adipamide (6,6 nylon). An efficient removal of the by-product(s) is important for efficiently producing a polycondensation polymer by the melt polycondensation process.
Various apparatuses and methods for producing polycondensation polymers by the melt polycondensation process have been known. For example, Examined Japanese Patent Application Publication No. 46-34083, Examined Japanese Patent Application Publication No. 50-19600 (corresponding to GB-1007302) and Examined Japanese Patent Application Publication No. 3-14052 disclose a horizontal agitation type polymerizer vessel equipped with a revolving shaft. In each of the above-mentioned publications, the horizontal agitation type polymerizer vessel equipped with a revolving shaft is used to renew the surface of the polymer by rotary agitation so as to promote the separation of the by-product(s) of the polycondensation reaction from the reaction system. However, this technique has the following problem. Generally, in order to remove the by-product(s) of the polycondensation reaction from the reaction system, the polycondensation reaction is conducted under a high vacuum. However, a polymerizer having a revolving shaft, in which the gap between the casing of the polymerizer and the revolving shaft is sealed, for example the horizontal agitation type polymerizer vessel mentioned above, has a problem in that air leakage is likely to occur at the sealed portion of the polymerizer. In addition, the quality of the produced polymer is lowered by the shear heat generated by the revolution of the revolving shaft.
Further, as polymerizers used for producing a polycondensation polymer, polymerizers which do not use a revolving shaft and, instead, allow a prepolymer to fall freely therein so as to effect the polymerization of the prepolymer during the free fall thereof have also been known. For example, Examined Japanese Patent Application Publication No. 48-8355 describes a polymerizer comprising a polymerizer casing containing therein a porous body extending in the substantially vertical direction, wherein the porosity of the porous body increases as from an upper end of the porous body toward a lower end of the porous body; a distribution device for the feedstock mixture fed into the polymerizer casing, which is disposed in the polymerizer casing at an upper portion thereof; an exhaustion means for removing gas generated during the reaction in the polymerizer casing, which is provided in association with the polymerizer casing; and a withdrawal means for the produced polymer, which is provided at a lower portion of the polymerizer casing. Unexamined Japanese Patent Application Laid-Open Specification No. 53-17569 describes an apparatus comprising a hollow body containing: a number of linear guides extending in the vertical direction; a nozzle for feeding a highly viscous prepolymer, which is provided above the linear guides; and a withdrawal means for the produced polymer, which is provided below the linear guides. Further, Examined Japanese Patent Application Publication No. 4-14127 discloses a continuous polycondensation method in which a prepolymer having an intrinsic viscosity of at least 0.1 is extruded through a slit orifice into a heated reactor vessel, and the resultant film of the prepolymer is held between two wires extending downwardly from the slit orifice and allowed to move contenuously in a downward direction. In the above-mentioned patent documents, it is attempted to remove efficiently the by-product(s) of the polycondensation reaction from the reaction system by increasing the surface area of the prepolymer during the fall thereof in the apparatus (i.e., by improving the surface renewal of the prepolymer during the polymerization). However, by the techniques disclosed in the above-mentioned patent documents, the surface renewal of the prepolymer is not always satisfactory and the polymerization rate is disadvantageously low.
Unexamined Japanese Patent Application Laid-Open Specification No. 63-104601 describes a method for removing volatile substances from a liquid by using an apparatus having disposed therein a belt conveyer, wherein a belt (carrier means) of the belt conveyer is moved at a predetermined speed, and a liquid to be treated is fed onto the belt at an upstream end portion of the belt conveyer, wherein the liquid forms a thin film on the belt and the volatile substances are removed by evaporation from the thin film of the liquid during the movement thereof on the belt conveyer. When this method is employed for the polycondensation reaction, the surface renewal of a prepolymer is improved as compared to that in the case of the above-mentioned apparatuses in which the polymerization of a prepolymer is effected during the fall of the prepolymer. However, since the belt conveyer in the above-mentioned apparatus needs to be driven by an outside power source, the apparatus necessarily has a sealed portion at which the gap between the casing of the apparatus and a means for transmitting the power provided by the outside power source to the belt conveyer is sealed. Therefore, as in the case of the horizontal agitation type polymerizer vessel mentioned above, this apparatus, having disposed therein a belt conveyer, has a problem in that air leakage is likely to occur at the sealed portion, and the air leakage causes a lowering of the quality of the produced polymer.
On the other hand, with respect to the melt polycondensation process for producing polycondensation polymers, it is widely known to effect a polymerization in the presence of an inert gas so as to remove by-product(s) from the reaction system. For example, Unexamined Japanese Patent Application Laid-Open Specification No. 6-206997 (corresponding to U.S. Pat. No. 5,384,389) describes a method in which an aromatic polycarbonate is produced by performing the equilibrium polycondensation reaction while continuously introducing an inert gas, together with a molten oligocarbonate, into a heated polymerizer under atmospheric pressure or under superatmospheric pressure in an amount of 1 m.sup.3 or more per kg of the oligocarbonate, and removing by distillation phenol or the like (which is by-produced in the equilibrium polycondensation reaction) in such a form as entrained by the inert gas. However, such a method (in which the production of an aromatic polycarbonate is conducted while reducing the partial pressure of the by-produced phenol in the polymerizer by using a large amount of the inert gas) has a problem in that, for recycling the inert gas used in the polymerization reaction, a large separation device is necessary to separate the phenol from the inert gas. Each of the above-mentioned Examined Japanese Patent Application Publication No. 4-14127 and Unexamined Japanese Patent Application Laid-Open Specification No. 63-104601 also discloses a method for producing a polyester under the flow of an inert gas. In this method also, it is necessary to use a separation device for removing the by-produced ethylene contained in the inert gas. Therefore, this method has the same problem as mentioned above in connection with Unexamined Japanese Patent Application Laid-Open Specification No. 6-206997.
Further, in Unexamined Japanese Patent Application Laid-Open Specification No. 8-325373, the present inventors proposed a method in which the polymerization of a prepolymer is conducted by allowing the prepolymer to fall along and in contact with the surface of a guide, wherein the ratio of the partial pressure of the aromatic monohydroxy compound contained in the inert gas to the partial pressure of the inert gas is controlled within such a specific range that the recovery of the inert gas can be conducted even without using a large apparatus. However, in this method, it is difficult to achieve a sufficiently high polymerization rate.
In conventional methods in which an attempt is made to produce a polycondensation polymer by efficiently withdrawing by-product(s) from the polymerization reaction system by using an inert gas, as in the case of the above-mentioned Unexamined Japanese Patent Application Laid-Open Specification No. 6-206997, Examined Japanese Patent Application Publication No. 4-14127 and Japanese Patent Application Laid-Open Specification No. 63-104601, the inert gas is continuously introduced into a polymerizer so as to reduce the partial pressure of the by-product(s) in the polymerizer, to thereby advance the polymerization reaction. In the above-mentioned conventional methods, however, for effectively increasing the polymerization rate, it is necessary to use a large amount of the inert gas.